1. Field of the Invention
The invention relates to a class D amplifier circuit. In particular, the invention relates to a PWM circuit in a class D amplifier.
2. Description of the Prior Art
The class AB amplifier and a class D amplifier and is both power saving and has better tone quality than the class A amplifier that provides lower distortion, but it has higher power consumption, and class B amplifier has lower power loss, but has crossover distortion, hence widely used in consumer stereo and audio-visual equipment at present. The amplifier's main difference is the class AB amplifier uses a linear action region to amplify signal by power transistor, a class D amplifier technology is used for the PWM to amplify signal.
Please refer to FIG. 1. FIG. 1 shows a basic block diagram of half-bridge class D amplifier. The class D amplifier comprises of an integrator 102, a comparator 104, a pre-driver 106, a power amplifier circuit 108 and a low-pass filter 110. As show in FIG. 1, an input end of the integrator is used in receiving an analog signal V1, and uses a resistor R couples to another input end and an output end of a power amplifier circuit.
Analog signal V1 generates a voltage V2 after going through the integrator 102. The integration result compares with a normal triangular wave by comparator 104. The triangular wave signal VTRI can be a sampling signal of the integrate result V2. Therefore, in order to get a better sampling result, the triangular wave VTRI frequency is usually larger than the best high frequency of the analog audio signal V1.
When V2 voltage is higher than VTRI, the output result of the comparator 104 can be a high level pulse wave signal; When V2 voltage is smaller than VTRI, the output result of the comparator 104 can be a low level pulse wave signal. The integrator 102 and the comparator 104 are usually called a PWM circuit. The original analog audio signal is able to transform into a PWM signal of digital type by the PWM circuit; the amplitude value of the original analog signal direct ratio to part of the high level of pulse width in the digital signal.
Then the output voltage V3 of the comparator 104 can be amplified by pre driver 106, and not only drives a power amplifier circuit 108, but also controls and turns on or off the power transistor M1, M2. The power amplifier circuit 108 is used for amplifying signal. The low-pass filter 110 connected to the output end of the power amplifier 108 comprises an inductance and a capacitor, and the low-pass filter 110 to filter a carrier wave in PWM signal, and return the wave of the analog audio signal. Therefore, class D amplifier provides the returned signal V4 to drive the amplify 112. Please refer to FIG. 2, which shows a wave sample of the V1, VTRI, V3 and V4.
Please refer FIG. 3. FIG. 3 shows a full-bridge class D amplifier circuit. In the example, the integrator 32 transmitted the integration result into the comparator 33A and 33B respectively after receiving the differential audio signal Vin+ and Vin−, after both output signals of the comparator go through the pre driver (34A, 34B), the power amplifier circuit (35A, 35B), and low-pass filter (36A, 36B), can be an analog signal used in driver the amplifier 38. The integrator 32 and pre driver 34A, 34B is also called a PWM circuit. As show in FIG. 3, the comparator 33A and 33B have an input end to receive the triangular signal VTRI, respectively.
With the explanations previously, both the half-bridge and the full-bridge class D amplifier need a triangular wave to be the standard of the PWM. In the prior art, the triangular wave signal usually is generated from a special circuit outside of the class D amplifier. As known by those skilled in the art, the amplitude and duty cycle of the triangular wave should be quite exact to avoid the inaccuracy of the voice output from the amplifier. Therefore, the triangular wave generation circuit usually is quite complex.
In addition, when the complex triangular wave generation circuit is integrated with the class D amplifier in one single chip, it occupies a large area of the chip, and the cost for producing said chip will be largely increased.